This invention relates generally to electronic non-contact distance measuring systems and the electronic circuitry employed therewith. More particularly, this invention relates to a novel electronic capacitive gap measuring transducer and a novel circuit termed a twin tee synthetic resonance circuit which may be used in conjunction with the measuring transducer.
There has been a continuing need for efficient and accurate devices capable of measuring gap widths or clearances without contact between spaced boundaries of a gap. For example, the difficulties of measuring the clearance gap between two plates has long plagued the art of metrology. The simple and known technique of inserting the "just fits" stacks of calibrated "feeler" shims has the disadvantage of contact inaccuracies as well as being a time consuming, non-electrical output measurement. Thus, the problem of verifying alignment of an automobile door often still requires a caliper measurement of clay daubs which have been squashed in the gap.
Prior art electronic devices are known to accomplish such contactless gap measurements. In these prior measurement systems, electromagnetic induction phenomena has often been employed to sense proximity or distance changes between a transducer and a metal object. Such measurement systems are useful in a wide variety of applications particularly where it is impossible or undesirable that there be physical contact with the object defining the distance to be measured. Other applications include pressure transducers, accelerometers, electronic micrometers, dimension comparators, bore gages, limit gages and liquid metal level detectors.
Previous electromagnetic induction measuring systems have not achieved the degree of accuracy and stability necessary for concise and accurate distance measurements. Certain limitations have restricted the development of these prior art systems, such as the difficulty in obtaining sufficient sensitivity and resolution over the effective measurement range of the system. This limitation results from the failure of the prior art systems to distinguish between the magnetic properties of the object and to compensate for these properties. Another limitation has been error caused temperature variations. Temperature changes cause impedance changes in the object and in the inductive distance measuring components of the system, and these impedance changes are reflected as a change in distance when in reality no such change may have occurred. A further problem with prior art systems has been that of non-linearity.
U.S. Pat. No. 4,160,204, which is assigned to the assignee hereof and incorporated herein by reference, relates to an improved non-contact measuring distance system which exhibits high sensitivity and resolution over the effective measurement range of the system, is virtually insensitive to variations in temperature and provides a high degree of linear relationship between the output provided and the distance to be measured. The measuring system of Patent No. 4,160,204 generally comprises a high frequency signal source, an inductive transducer and a reference impedance (both connected in a signal phase network and to the source), and a means for comparing the signals from the transducer and the reference impedance to provide an output related to the distance between the transducer and the object. A circuit element such as a capacitor is connected in parallel with the transducer for the purpose of enhancing the sensitivity and resolution of the system, for significantly reducing or effectively eliminating errors caused by temperature variations in the transducer or in the object measured, and for providing a high degree of linear relation between the output provided and the distance measured.
While well suited for its intended purposes, the inductance sensor measuring system of Patent 4,160,204 does suffer from the drawback that its measurement can be effected by the particular material which define the gap boundaries. This sensitivity to gap boundary composition may adversely affect the accuracy of the measurement system. Also, because the system is based on inductive measurement, the prior art system of Patent 4,160,204 cannot measure gaps where the boundary are not metallic (e.g. such as plastic materials).